1. Field of the Invention
The present invention relates generally to anti-tumor vaccines and, more particularly, to the use of gene therapy to augment immunological functions in response to anti-tumor vaccines.
2. Background Information
Recent advances in our understanding of the biology of the immune system have lead to the identification of cytokines as important modulators of immune responses. Cytokines produced by lymphocytes are termed lymphokines. These agents mediate many of the immune responses to tumors. Several cytokines have been produced by recombinant DNA methodology and evaluated for their anti-tumor effects.
The administration of lymphokines and related immunomodulators has produced some positive responses in patients with various types of neoplasms. However, current cytokine administration is frequently associated wit toxic effects that limit the therapeutic value of these agents. For example, interleukin-2 (IL-2) is an important lymphokine in the generation of anti-tumor immunity. In response to tumor antigens, a subset of lymphocytes, helper T cells, secrete a small amount of IL-2, which acts locally at the site of tumor antigen stimulation to activate cytotoxic T cells and natural killer cells that mediate systemic tumor cell destruction.
In addition to immunostimulatory agents such as cytokines, which positively regulate immune effector functions, there also exist molecules that exhibit imunosuppressive activity. These immunosuppressive agents, if aberrantly regulated, can have detrimental effects on the induction of systemic immunity. For example, one or more isoforms of transforming growth factor-.beta. (TGF.beta.) can be an immunosuppressive agent that is secreted by many tumor types. Culture supernatants from tumor cells that secrete an immunosuppressive form of TGF.beta. can reduce tumor specific cytotoxicity in vitro. In these in vitro cytotoxicity assays, inhibition of TGF.beta. activity by expression of antisense TGF.beta. can enhance tumor cell cytotoxicity.
Due, in part, to endogenous concentrations of immunosuppressive agents in a subject having a cancer, it cannot be predicted whether inhibiting the secretion of an immunosuppressive agent by the tumor cell will render the tumor cell immunogenic in vivo. For example, TGF.beta. secreted by cancer cells can circulate throughout a cancer patient and generally immunocompromise the patient. As a result, administration of tumor cells that are substantially similar to the patient's cancer cells and that are genetically modified to prevent the expression of an immunosuppressive agent would not necessarily be expected to stimulate the patient's immune response against the cancer cells. Similarly, the results of in vitro models of anti-tumor immune responses do not reliably predict the outcome of related immune system manipulations in vivo.
The modulation of cytokine concentrations has been attempted as a means to enhance a cancer patient's immune response toward target cancer cells. For example, intravenous, intralymphatic or intralesional administration of IL-2 has produced clinically significant responses in some cancer patients. However, severe toxic effects such as hypotension and edema limit the dose and efficacy of intravenous and intralymphatic IL-2 administration. The toxicity of systemically administered lymphokines is not surprising as these agents mediate local cellular interactions and normally are secreted only in very small quantities. In addition, intralesional administration of IL-2 can be difficult to accomplish and can cause significant patient morbidity.
To circumvent the toxicity of systemic cytokine administration, an alternative approach involving cytokine gene transfer into tumor cells has produced anti-tumor immune responses in several animal tumor models. In these studies, the expression of cytokines following cytokine gene transfer into tumor cells resulted in a reduction in tumorigenicity of the cytokine-secreting tumor cells when implanted into syngeneic hosts. Reduction in tumorigenicity occurred using IL-2, gamma-interferon or interleukin-4. In studies employing IL-2 gene transfer, the treated animals also developed systemic anti-tumor immunity and were protected against subsequent tumor cell challenges with unmodified parental tumor cells. Similar inhibition of tumor growth and protective immunity also was demonstrated when immunizations were performed with a mixture of unmodified parental tumor cells and tumor cells that were genetically modified to express IL-2. No toxicity was associated with localized lymphokine transgene expression in these animal studies.
Cytokines also have been expressed in heterologous cell types such as fibroblasts, which were coinjected into a cancer patient with the patient's own cancer cells. Coinjection of cytokine-expressing cells yielded a similar induction of systemic anti-tumor immunity as produced by cytokine-expressing tumor cells. Nevertheless, while these gene transfer procedures can provide significant anti-tumor immunity compared to other methods, a significant fraction of the patients do not respond optimally to such therapy. Thus, there exists a need to provide more effective methods to prevent or reduce the severity of a cancer in a patient. The present invention satisfies this need and provides related advantages as well.